Add an :if_exists option to drop_table

If set to `if_exists: true`, it generates a statement like:

    DROP TABLE IF EXISTS posts

This syntax is supported in the popular SQL servers, that is (at least)
SQLite, PostgreSQL, MySQL, Oracle and MS SQL Sever.

Closes #16366.

activerecord/CHANGELOG.md

@@ -1,3 +1,18 @@
+*   Introduce the `:if_exists` option for `drop_table`.
+
+    Example:
+
+        drop_table(:posts, if_exists: true)
+
+    That would execute:
+
+        DROP TABLE IF EXISTS posts
+
+    If the table doesn't exist, `if_exists: false` (the default) raises an
+    exception whereas `if_exists: true` does nothing.
+
+    *Cody Cutrer*, *Stefan Kanev*, *Ryuta Kamizono*
+
 *   Don't run SQL if attribute value is not changed for update_attribute method.
 
     *Prathamesh Sonpatki*

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -380,7 +380,7 @@ def rename_table(table_name, new_name)
       # it can be helpful to provide these in a migration's +change+ method so it can be reverted.
       # In that case, +options+ and the block will be used by create_table.
       def drop_table(table_name, options = {})
-        execute "DROP TABLE #{quote_table_name(table_name)}"
+        execute "DROP TABLE#{' IF EXISTS' if options[:if_exists]} #{quote_table_name(table_name)}"
       end
 
       # Adds a new column to the named table.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -502,7 +502,7 @@ def rename_table(table_name, new_name)
       end
 
       def drop_table(table_name, options = {})
-        execute "DROP#{' TEMPORARY' if options[:temporary]} TABLE #{quote_table_name(table_name)}#{' CASCADE' if options[:force] == :cascade}"
+        execute "DROP#{' TEMPORARY' if options[:temporary]} TABLE#{' IF EXISTS' if options[:if_exists]} #{quote_table_name(table_name)}#{' CASCADE' if options[:force] == :cascade}"
       end
 
       def rename_index(table_name, old_name, new_name)

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -112,7 +112,7 @@ def table_exists?(name)
         end
 
         def drop_table(table_name, options = {})
-          execute "DROP TABLE #{quote_table_name(table_name)}#{' CASCADE' if options[:force] == :cascade}"
+          execute "DROP TABLE#{' IF EXISTS' if options[:if_exists]} #{quote_table_name(table_name)}#{' CASCADE' if options[:force] == :cascade}"
         end
 
         # Returns true if schema exists.

activerecord/test/cases/migration/change_schema_test.rb

@@ -403,6 +403,17 @@ def test_column_exists_on_table_with_no_options_parameter_supplied
         end
       end
 
+      def test_drop_table_if_exists
+        connection.create_table(:testings)
+        assert connection.table_exists?(:testings)
+        connection.drop_table(:testings, if_exists: true)
+        assert_not connection.table_exists?(:testings)
+      end
+
+      def test_drop_table_if_exists_nothing_raised
+        assert_nothing_raised { connection.drop_table(:nonexistent, if_exists: true) }
+      end
+
       private
       def testing_table_with_only_foo_attribute
         connection.create_table :testings, :id => false do |t|